On wind farms, the power that is generated by a plurality of wind energy installations is fed into an internal wind farm network. The power that is fed into the internal wind farm network is fed into a power supply network at a substation after undergoing transformation as required. In order to maintain the quality of the power supply in the power supply network, the power that is fed into the power supply network by the wind farm must meet certain specifications, for example, with respect to the power factor, that is, the ratio between the real power and the reactive power. This is monitored by a farm control system known as a “farm master,” which is able to influence individual control devices of the wind energy installations in such a way that the sum of the power generated by the wind energy installations fulfills the requirements resulting from the power supply network.
The real power generated on a wind farm cannot be freely determined by the farm master, but rather is dependent on the wind speed and the design output of the individual wind energy installations. However, the farm master can be used to determine the amount of reactive power generated by the wind farm.
Power losses occur in the lines of the internal wind farm network. These losses can be reduced by varying the reactive power. Prior art describes measures for doing this, some of which work in contrary directions.
For example, if the wind energy installations are at a standstill, but there is a requirement for reactive power, passive components can be used to meet the reactive power requirement, instead of the converters in the wind energy installations, which are subject to operating losses.
Furthermore, at times when the wind energy installations are operating at high capacity, it is possible to select the maximum possible voltage in the internal wind farm network at the substation. This results in a reduction of the losses in the cables in the internal wind farm network. This is particularly advisable when the wind energy installations are operating at high capacity, since cable losses dominate in this case. In contrast, when the wind energy installations are operating at low capacity, the minimum possible voltage should be selected in the internal wind farm network, since transformer losses dominate in this case. These losses can be reduced by selecting a low voltage in the internal wind farm network. DE 10 2008 048 258 Al describes a method for changing the voltage in the internal wind farm network depending on the amount of power generated by the wind energy installations.
Furthermore, it is possible to minimize the distance over which the power in the internal wind farm network must be transported. This allows the distance over which the reactive power must be transported to be minimized, thus reducing cable losses. However, the potential for reducing losses in the internal wind farm network in this way is limited.
European patent application EP 2 108 828 A2 describes a process for operating a wind farm in which a farm control system is designed such that the reactive power required to meet the requirements of the power supply network is generated primarily in the wind energy installations that are near the node, whereas the wind energy installations that are more distant make a smaller contribution to the generation of reactive power. This ensures that a large portion of the reactive power must only be transported over a short distance between the wind energy installation and the node in the internal wind farm network, and that a smaller portion of the reactive power must be transported over a longer transport route. Overall, there is a reduction of loss due to reactive power transport in the internal wind farm network. According to this prior art, the farm control system transmits individual target values for real and reactive power to every wind energy installation. The farm control system must therefore be designed to be able to solve a complex system of equations. Solving the system of equations takes considerable time, causing a delay in the adjustment of the real and reactive power generated on the wind farm when changes occur in the power supply network. During this delay, it cannot be ensured that the power that is fed into the power supply network by the wind farm meets the requirements of the power supply network.